The present invention relates to a safety switching apparatus for failsafe disconnection of an electrical load, such as an electrical motor or electromagnetic valve in an automatically operated installation.
Safety switching apparatuses in terms of the present invention are typically used to shut-down a dangerous machine or installation in a failsafe manner when this is necessary in order to protect people. The safety switching apparatuses typically monitor signals generated from or via emergency-off buttons, guard door switches, light barriers, light grids or other safety signaling devices. The safety switching apparatuses are designed to interrupt a power supply path to the monitored machine or installation as a function of or depending on these signals.
Depending on a machine's or an installation's potential for danger, there are different safety categories stipulating what requirements a safety switching apparatus needs to meet for a particular use. For example, European standard EN 954-1 defines four safety categories, with category 4 imposing the highest safety requirements in terms of intrinsic failsafety of the safety switching apparatus. Apparatuses which comply with category 4 are usually designed with multichannel redundancy. In such a case, the signals must also be in multichannel-redundant form, i.e. they must include at least a first and a second signal having a correlated information content.
In addition, category 4 requires the safety switching apparatus to identify a short on the signal lines which transmit these signals, because a short means that single-fault safety is lost. For applications below category 4, however, it is possible to dispense with short-circuit identification, which simplifies the wiring of the safety switching apparatus and makes installation less expensive.
A prior art safety switching apparatus is sold by the assignee under the brand name PNOZ X8P. This prior art safety switching apparatus can be used either with or without short-circuit identification, with the wiring connection of the signal lines for supplying these signals being different in the two cases. For an application with short-circuit identification, the second channel of the signaling device (which produces the second signal) has to be connected to different terminals than for an application without short-circuit identification. To allow flexible use, the known safety switching apparatus therefore requires a relatively large number of connection terminals.
German Patent Publication DE 44 23 704 C1 discloses a safety switching apparatus wherein a first signal is supplied to the control circuit of a first switching element via a first terminal, while a second signal is routed to the control circuit of a second switching element via a second terminal. Due to the fact that the control circuits of the two switching elements are further connected to a common ground potential and the signals have a positive and a negative signal level relative to the common ground potential, it is possible to identify a short between the two signals. However, this prior art safety switching apparatus does not allow operation without short-circuit identification, and it is therefore limited in terms of the flexibility of use. Furthermore, this known safety switching apparatus requires an AC supply, which also restricts the opportunities for use.